It has been known that a trench gate is arranged for improving the withstand voltage of an insulated gate bipolar transistor (IGBT). The testing of withstand voltage of an insulated film or measurement of a margin at a dummy trench gate is performed by applying a predetermined potential between a gate electrode and an emitter electrode of the dummy trench gate as disclosed in Patent Document 1.
However, it is common that the gate electrode is connected to the emitter electrode during manufacturing an element and the dummy gate trench is fixed to have the same potential as the emitter potential of the IGBT. Accordingly, the testing of withstand voltage for the dummy trench gate cannot be operated after completing the formation of the upper layer (i.e., the outer portion of the surface of a semiconductor substrate) of the IGBT.
Since the distributions of electrical field in a variety of elements for constituting an IGBT are different in the process of forming the upper layer and the completion of forming the upper layer, it is possible that the withstand voltage cannot be accurately measured in a conventional method.
[Patent Document 1] Japanese Patent Application Publication No. 2013-251466